The present invention relates generally to mobile antennas, and more particularly to a mobile antenna having a radiating element of adjustable effective length whereby the antenna can be tuned to a desired operating frequency without varying the physical length of the radiating element.
In recent years the increased use of personal mobile communications equipment, particularly that intended for use on the Citizens Band, has created a need for high-frequency antennas suitable for mounting on the exterior of automobiles or other vehicles. Such antennas are ordinarily required to operate within a desired range of transmitting frequencies, and the length of the radiating portion of such antennas is ordinarily made adjustable to enable the antenna to operate most efficiently in this range.
Where the length of the radiating portion has been adjustable in prior art mobile antennas, this has typically been accomplished by cutting the radiating element to length and then seating the element in a bore provided in an electrically conductive end fitting on the top of the base. The radiating element is then typically locked in the end fitting by means of a set screw in the end fitting perpendicular to the bore in which the radiating element is received. This arrangement has the disadvantage of requiring the user to cut the radiating element, which is not only time consuming, but also precludes the user from tuning the antenna to a lower operating frequency should that be required in the future as a result of a change in operating frequency or an error in his original length calculation. Furthermore, the exposed set screw utilized to lock the radiating element in position invites tampering with the antenna and the tuning adjustment.
Accordingly, the object of the present invention, generally stated, is to provide a new and improved mobile antenna in which the length of the radiating element can be adjusted without changing the physical length of the element.
Another object of the present invention is to provide a new and improved mobile antenna wherein the means for adjusting the length of the radiating element are inaccessible from the exterior of the antenna when the antenna is mounted for use.